Has he the will to seize the crown?

The Environment Secretary, nicknamed Brains by Alastair Campbell, is seen as the only man who can stop Gordon Brown succeeding Tony Blair. But sooner or later, he will have to decide if he is really up to the challenge

When David Miliband stepped aboard a cross-Channel ferry shortly before Easter, he did so with more than a busy cabinet minister's ordinary sense of relief at the prospect of a week away with his family. The Secretary of State for the Environment was also looking forward to a respite from months of escalating pressure to take the most important decision of his political life: whether to challenge the front-runner, Gordon Brown, for the Labour leadership and the keys to 10 Downing Street.

Returning home last week, the boyish-looking 41-year-old Miliband was clearly no closer to throwing his hat into the ring, nor to closing off that option altogether. He was 'flattered', he said, by the pleas from an assortment of 'stop Gordon' figures to offer himself for the top job. But he was not about to allow himself to be 'seduced - one way or the other'.

His emergence as the one potentially serious challenger to Brown for Tony Blair's crown marks the latest stage in an extraordinary political ascent. In 1994, having worked at New Labour's favourite think-tank, the Institute for Public Policy Research, he joined Blair, newly elected as opposition leader, as his top policy adviser. It was a role he continued in Number 10 when Labour ousted the Tories in 1997.

Four years later, he became MP for the safe Labour seat of South Shields and was then given two junior minister roles. It was only in the ministerial reshuffle last year that Miliband got his first major cabinet job, the high-profile brief for the environment. Now, in a turn of events that, friends say, has astonished no one more than Miliband himself, he is being mentioned as a possible successor to his former boss as Prime Minister.

Blair has been careful to steer clear of public comment on the prospect of a Miliband challenge. But, as The Observer revealed last month, he has privately told at least one close ally that Miliband 'can win' if he throws himself fully into a challenge. And if there is any doubt about how highly he rates the talents of his former policy chief, one clue may lie in a remark Blair made in an interview at the high point of the England side's tilt at the 2006 World Cup. Did the Prime Minister have a 'Wayne Rooney figure' in his cabinet? he was asked. His answer: David Miliband.

Perhaps the most remarkable aspect of Miliband's rise is how few enemies he has collected along the way. Even within the Brown camp, it's hard to find an MP with a nasty word to say about him, in part, perhaps, because Miliband's younger brother, Ed, is one of the Chancellor's close aides and proteges.

'Nice' is not a word often attached to politicians these days, of whatever rank or party. But it is routinely ascribed to the Environment Secretary, along with a handful of other consensus compliments: reflective, thoughtful, private, protective of his family life, funny - and, above all, very smart. During Miliband's time in Downing Street, Alastair Campbell used to call him, simply, 'Brains'.

Still, as he ponders whether to put himself forward, the key question marks concern other qualities much more familiar in the rough and tumble of Westminster politics: natural media presence, steel, toughness, raw ambition.

Officially, when asked pointblank whether he's going to run, Miliband recites an elegantly inconclusive formula: 'Gordon Brown is an excellent Chancellor and an excellent Prime Minister-in-waiting.' Yet privately, friends say, he has been considering his options, most recently on the Easter break with his concert violinist wife, Louise Shackelton, and their two-year-old adopted son, Isaac. 'If David had to decide today, I'm sure the answer would be no,' one friend says, but adds: 'He does think that he could win if he did stand.'

Miliband is the son of Jewish child refugees from the Holocaust, who saw the effects of fascism at first hand and became passionately socialist. It is a credo that has led his mother, Polish-born Marion Kozak, to support a range of left-wing Jewish political groups. His Belgian-born father, the late Ralph Miliband, became one of the leading Marxist theoreticians of his generation.

He told a Jewish Chronicle interviewer recently that he had inherited from his parents 'a commitment to equality, social justice, freedom of expression and solidarity ... if you try to analyse where I stand politically, it would be very hard to explain that without reference to my roots.'

But like much else about Miliband, it is not quite that simple.

First at the IPPR, then in Number 10, Miliband became not only a supporter but a co-architect of Blair's New Labour project. But as a constituency MP and particularly in his first cabinet role as Minister for Communities and Local Government, he began increasingly to champion a new style of government that envisaged loosening the reins of Westminster and forging a partnership with local communities. Setting out his political vision in a recent New Statesman article ranging well beyond his environment brief, Miliband spoke of a new 'I can' spirit in society and said that Labour's challenge must be to embrace and reflect it.

The intellectual flowering came when he was well into his teens. Miliband was educated at a comprehensive in Primrose Hill, north London. He notched up indifferent A-level results, but went on to achieve a first, like David Cameron, at Oxford.

As Blair's policy adviser, he soon impressed those in the inner circle as bright, articulate, energetic, personable - and young. When Blair won the 1997 election, he briefly hesitated over whether to bring Miliband into Downing Street as his policy chief. 'The question was whether he was seasoned enough, senior enough,' recalls a friend. But with Peter Mandelson, in particular, arguing in favour, Miliband was brought into the government team, and quickly excelled. 'The lesson,' recalls Mandelson, 'is don't underestimate David.'

But don't, he might have added, oversimplify him, either. If Miliband does decide to seek the Labour leadership, close friends say one of the first things he is likely to do is to distance himself politically from Mandelson and other core Blairites. 'There is a sense among all the younger figures that the Blair-Brown tribal battle is something for the previous generation,' one friend explains.

In fact, one of the main reasons Miliband has so far resisted the idea of standing is the possibility that a leadership contest would spark a final, bloody battle between Blairites and Brownites and fatally damage the party's chances of defeating Cameron for a record fourth Labour election victory.

There are other reasons to hesitate. Miliband's public demeanour has evolved from IPPR-style policy wonk to an increasingly assured public performer. Still, even friends acknowledge, it is not a role that comes naturally. His much-anticipated speech at last year's Labour conference fell nowhere near as flat as fellow potential leadership candidate Alan Johnson, but poetry it wasn't.

Miliband also worries about the effects of a leadership run on his young family. He and his wife still bear the scars of media coverage of their adoption of Isaac. Newspapers questioned why they chose to adopt Isaac in the US (Louise grew up in the US, holds an American passport, and both she and David were concerned about the potential complications for relations with his birth parents of the life of a minister constantly in the public eye).

For all his passion for politics, Miliband still enjoys the ability to cheer on Arsenal and, more frequently, watch his wife performing with the London Symphony Orchestra.

As Blair enters his final weeks in office, two men, above all, will be watching to see which way Miliband jumps. The first is Gordon Brown, whose supporters have been working hard to win the backing of a critical mass of Labour MPs and thus nip in the bud any thoughts of an all-out challenge.

The other interested spectator is David Cameron. The Tories have gone to some lengths to create the public impression that their electoral enemy of choice is Brown. Privately, the picture is slightly more nuanced. Cameron's inner circle recognises that Brown would assume the leadership with a strong record as Chancellor, a proven mastery of policy and an image of seriousness and solidity. But Miliband, young, personally untainted by the decade-long Brown-Blair civil war and English, does genuinely worry them.

Now it is over to Miliband. Still minded not to stand, friends say, it would be wrong to assume absolutely that he will not. The mood in the party when Blair goes, the extent of Labour's meltdown in next month's local, Scottish and Welsh elections, the opinion poll ratings from Brown - all could play a part in his final decision.

But the Prime Minister, with his instinctive sense of the bigger political picture, may inadvertently have put his finger on the crucial issue in remarks to his inner circle about a possible Miliband candidacy. 'If Miliband is to win,' he remarked, 'he really has got to want it.'

The Miliband Lowdown

Born David Wright Miliband, 15 July 1965, London. Son of the late Ralph Miliband, a Belgian Jewish refugee and leading Marxist writer, and Polish-born Marion Kozak. Married, with one child.

Best of times Triumphant arrival into Downing Street as Tony Blair's policy adviser after the 1997 election ended Labour's 18 years in opposition.

Worst of times The intrusive media assault surrounding the American adoption of the Milibands' son, Isaac.

What he says 'Politics requires many virtues - organisation, ideas, resolution, luck. But chief among them is the hardest to define: that elusive sense of being in tune with the times.'

What others say 'David Miliband is ferociously bright, but it's matched by people skills of the first order... he is one of my favourite people.' Former Education Secretary Estelle Morris